PRELIMINARY HYDROGEOCHEMICAL STUDY OFA FRACTURED AQUIFER SYSTEM IN URUGUAY: PUNTA ESPINILLO CASE STUDY

Abstract

Punta Espinillo area is located in the Northwest of Montevideo district/Uruguay, besides the right margin of Santa Lucia River, in the mouth of it, into the Río de la Plata river. The territory structure is built in small plots which vary from 3 to 5 hectares. This zone is characterised by an intense agronomic activity, which produces a large amount of the vegetables consumed in Montevideo City. It has a high demand of water for irrigation systems, which has not been rationally planned. Nowadays, in every plot there is a well, and some are located in the same fracture. The water is extracted from a fractured aquifer system that has exceptional characteristics because of its water amount. However, its intense exploitation generated high drawdowns in wells, even coming to the extreme of their complete depletion. In connection, there is an increase of ionic concentrations of sodium, chloride, sulphates and nitrates well above of the national and international sanitary standards, which ones are suspicious of been associated with non appropriate agronomic practices and the possibility of marine water intrusion. These hypotheses were tested with the study of chemical analyses represented by geochemical contour maps and some geochemical indexes. On one hand, the sodium and chlorates geochemical contour maps (show that there is an increase in their concentration in wells who are located too far away from the Rio de la Plata river, to have a connection with it. On the other hand, the virtual potenciometric contour maps show that the water flow directions are NS, to the Rio de la Plata. In addition, the geochemical indexes calculated were Mg++/Ca++, K+/Na+ y Cl-/HC03-. These relations show that all the waters has a continental origin in opposite to the suspected marine water intrusion. On the other hand, in relation to nitrates, all the wells have very high concentrations, some of them above 300 mg/L. The higher concentrations are associated with plots which generally use natural animal fertilisers. Theses concentrations of sodium, chlorates and nitrates qualify theses waters as non drinkable, for the international standards, and non acceptable for the national standards. In relation with irrigation, they have severe restrictions due to of the risk of alcalinization and salinization of the soils.